It is widely believed that balancing a heating system increases boiler efficiency by reducing the boiler return temperature and increasing the condensing effect. Unfortunately, balancing does NOT increase condensing at the boiler, at least not in the way you would intuitively think and not with typical domestic modulating systems.
Getting the correct temperature drop across a system is done by controlling the speed of the pump, this is proven by the 'mass flow rate' (article to follow). Modern modulating boilers have 'burner linked pumps', these automatically adjust the pump flow rate to match the amount of heat energy being put in.
When a boiler puts in half the amount of heat or gas, the pump speed halves. This does two things, it keeps our delta T nice and wide to increase condensing, and exponentially reduces pump power consumption.
The problem you have, if you don't balance heating systems, is that the nearer radiators will take more flow (overflow), and further radiators less flow (underflow) as water takes the path of least resistance.
Intuitively, the nearer radiators having an 'overflow' would increase the return temperature to your boiler because the water does not have enough time to give up its heat. However, what also must be remembered is that other radiators with 'underflow' will cool the return temperature back down.
The following diagram shows an unbalanced 42kw heating system running at 30lpm, with 70°c flow temperature.
Room/rad A+B has an overflow, this only has a small temperature drop across the radiator meaning the return temperatures are 58 and 53°c, rather than the ideal 50°c.
Room/rad C has 1/5th of the flow rate, which is correct in this situation and so has the ideal 50°c return.
Room/rad D+E have an underflow, this has a small temperature drop across the radiator meaning the return temperatures are 35 and 25°c.
When these returns tee-in together they average their temperature relative to their flow rate (do the maths on the above diagram if you wish).
When the 2 litres per minute from zone/rad E tee-in to the 4 litres per min from rad D they average at 31.7°c.
When this meets the 6LPM at 50°c from radiator C the average return temperature is boosted to 40.8°c.
Radiator B then adds 8LPM at 53°c boosting the flow temperature to 45.7°c.
Finally, radiator A adds 10LPM at 58°c boosting the return to the target 50°c.
To further explain this let's imagine that we opened up a bypass between the flow and return pipe before the first radiator A. A certain volume of water would return back to that boiler at 70°c. This would slightly increase the return temperature to the boiler, the flow temperature would try to increase, so the boiler would modulate down and further reduce its pump speed.
The result is that the flow and return again end up at 70°c flow and 50°c return, and/or cycle as it struggles to match the heat load. This can be a complex thing to understand but if you play around with the mass flow rate equation you will find this is correct (article to follow).
So rather than increasing the return temperature, it would instead limit the boiler output.
Of course, in real life, nothing is exactly like the theory. There are a few things that may influence this and show an increased return. For example, if the boiler has reached its minimum flow rate of the boiler or pump, and the minimum output of the boiler. If the pump or boiler reaches its minimum output (which will happen often as boilers are generally hugely oversized) the return will increase and show an increased return or reduced DT, and cause the boiler to cycle.
Once you have balanced the system, this cycling will decrease. However, if you try to improve the reduced delta t by decreasing the total flow around the system, all that will happen is that the boiler's internal bypass will open and create yet another bypass! Not to mention increase pumping costs.
The only way (according to the theory of physics at least) that balancing can increase boiler efficiency is that it may prevent you from overheating some zones, in order to get other zones up to temperature. Conversely, however, it may also mean that some zones go under-heated while your main reference zone is warm enough. We would argue that this in effect is not boiler efficiency, but system efficiency.
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